This invention relates to thermal ink jet nozzle arrays, and particularly to arrays of modular printhead units arranged in two rows on opposite sides of a structural bar, with the units being staggered so that they combine to form a line of ink drop impact areas extending across the full width of a page on which printing is to be effected.
By "thermal ink jet" in this specification, it is meant that process by which individual drops of ink are ejected from a nozzle by heating the ink in communication with the nozzle, so that some of it vaporizes to form a transitory bubble which pushes a column of ink towards the nozzle. Matters are so arranged that the ink at the end of the column breaks off to form an ink droplet which travels under its own momentum towards a sheet of paper or other copy medium on which drops of ink are intended to fall, with the impact areas partially overlapping so that they form characters or other marks of desired shape. The ink is usually supplied to a plurality of channels, each terminating in a nozzle, from a common reservoir, with each channel being in thermal communication with a selectively energized resistor which produces the bubble in the channel at a precisely chosen time.
Thermal ink jet printheads are presently made and used in units containing 10 to 200 individual nozzles at a linear density of about ten nozzles per millimeter. The largest such printhead that it is presently practical to make with reasonable yield is of the order of 10 to 20 mm long. Such printheads are scanned across the medium to be marked (usually of sheets of paper) in order to print the entire page. Maximum drop ejection repetition frequency, as well as over-scan time and turn-around time, limits the print speed to about four pages per minute at a resolution of 12 dots per mm. Fabrication of a page-width print bar enables the print speed to increase to 10 to 100 pages per minute, by increasing the number of nozzles which may be made to eject ink drops at the same time, as well as by eliminating time wasted at the ends of scan lines. Assembly of a page-width print bar requires the precise location of several printhead modules. Of course it is not essential that a multi-module print bar be of page-width. In some applications, such as wide plotters, it may be desirable to increase plotting speed higher than is possible from the single printhead unit, but not necessary to use a print bar that extends across the full width of the plotter. In such a case, a multi-module printhead could be scanned across the paper. It may also prove advantageous, when wishing to print bi-color or multi-color images, to mount several printhead modules precisely on a bar, with each module being dedicated to ink of a specified color.
A page-width thermal ink jet bar will be composed of several printhead modules. These modules must be accurately positioned with respect to each other so that the line of picture elements (pixels) produced by printed droplets from neighboring modules show no seams, and the pixels appear to be produced by one continuous line of uniformly-spaced ink drop nozzles. It is further advantageous if the modules are replaceable, so that if one were defective it would not cause the entire print bar to be rejected. One way to achieve the two objectives of precise registerability and replaceability would be to build complex adjustment capability into the print bar substrate, as is disclosed in U.S. Pat. No. 4,559,543 to Toganoh et al. in which these adjustment features are labelled 107 and 108 in FIG. 1 of the patent. The disadvantage of this approach is the complexity, and therefore the cost of the print bar. The reason that the adjustment capability is needed in '543 is that there is no provision on each module for a precise location surface relative to the nozzles. For example, it is the base plate 202 (FIG. 2) which is shown as being in contact with the page-width substrate. Neither the base plate nor the adhesive joint to the thermal ink jet die or subunit have sufficiently precise thickness to ensure that the ink jets from adjacent modules would line up adequately to form a precise line of pixels, with no overlapping and no gaps.